NOTE: These are the main Left 4 Dead cheats that are usually applied by pressing down the right key combination. Please find the instructions on how to use these cheats below.

Developers Console Cheats
At the main menu, select "Options," then "Keyboard/Mouse." Enable "Allow Developers Console." During gameplay, press "~" to access the console. Type in "Sv_Cheats 1" and press "Enter" to enable the cheats. Then, enter any one of the following:

NOTE: As their name suggests hints are not exactly Left 4 Dead cheats, and are usually just simple (but very useful) tips on how you can unlock secret level, features, find hidden areas and such. Hopefully these will make playing Left 4 Dead even more entertaining.

Steam Achievements
Complete each requirement to receive the achievements.

A RIDE DENIED: Kill a Jockey within 2 seconds of it jumping on a Survivor.
A SPITTLE HELP FROM MY FRIENDS: As the Spitter, spit on a Survivor being choked by a Smoker.
ACID REFLEX: Kill a Spitter before she is able to spit.
ARMORY OF ONE: Deploy an ammo upgrade and have your team use it.
BACK IN THE SADDLE: As the Jockey, ride the Survivors twice in a single life.
BEAT THE RUSH: In a Survival match, get a medal only using melee weapons.
BRIDGE BURNER: Survive the Parish campaign.
BRIDGE OVER TREBLED SLAUGHTER: Cross the bridge finale in less than 3 minutes.
BURNING SENSATION: Ignite 50 Common Infected with incendiary ammo.
CACHE AND CARRY: Collect 15 gas cans in a single Scavenge match.
CHAIN OF COMMAND: Kill 100 Common Infected with the chainsaw.
CL0WND: Honk the noses of 10 Clowns.
CLUB DEAD: Use every melee weapon to kill Common Infected.
CONFEDERACY OF CRUNCHES: Finish a campaign using only melee weapons.
CRASS MENAGERIE: Kill one of each Uncommon Infected.
DEAD IN THE WATER: Kill 10 swampy Mudmen while they are in the water.
DISMEMBERMENT PLAN: Kill 15 Infected with a single grenade launcher blast.
FRIED PIPER: Use a Molotov to burn a Clown leading at least 10 Common Infected.
FUEL CRISIS: Make a Survivor drop a gas can during overtime.
GAS GUZZLER: Collect 100 gas cans in Scavenge.
GAS SHORTAGE: Cause 25 gas can drops as a Special Infected.
GONG SHOW: Prove you are stronger than Moustachio.
GUARDIN' GNOME: Rescue Gnome Chompski from the Carnival.
HEAD HONCHO: Decapitate 200 Infected with a melee weapon.
HEARTWARMER: In a Versus match, leave the saferoom to defibrillate a dead teammate.
HUNTING PARTY: Win a game of Scavenge.
LEVEL A CHARGE: Kill a Charger with a melee weapon while they are charging someone.
LONG DISTANCE CARRIER: As the Charger, grab a Survivor and carry them over 80 feet.
MEAT TENDERIZER: As the Charger, grab a Survivor and smash them into the ground for a solid 15 seconds.
MIDNIGHT RIDER: Survive the Dark Carnival campaign.
PRICE CHOPPER: Survive the Dead Center campaign.
QUALIFIED RIDE: As the Jockey, ride a Survivor for more than 8 seconds.
RAGIN' CAJUN: Survive the Swamp Fever campaign.
ROBBED ZOMBIE: Collect 10 vials of Boomer vomit from infected CEDA agents you have killed.
RODE HARD, PUT AWAY WET: As the Jockey, ride a Survivor and steer them into a Spitter's acid patch.
SCATTERING RAM: As the Charger, bowl through the entire enemy team in a single charge.
SCAVENGER HUNT: Stop the enemy team from collecting any gas cans during a Scavenge match.
SEPTIC TANK: Use a bile grenade on a Tank.
SHOCK JOCK: Revive 10 dead Survivors with the defibrillator.
SOB STORY: Navigate the sugar mill and reach the safe room without killing any Witches.
SPIT HAPPENS: As the Spitter, hit every Survivor with a single acid patch.
STACHE WHACKER: Prove you are faster than Moustachio.
STILL SOMETHING TO PROVE: Survive all campaigns on Expert.
STRENGTH IN NUMBERS: Form a team and beat an enemy team in 4v4 Versus or Scavenge.
TANK BURGER: Kill a Tank with melee weapons.
THE QUICK AND THE DEAD: Revive 10 incapacitated Survivors while under the speed boosting effects of adrenaline.
THE REAL DEAL: Survive a campaign on Expert difficulty with Realism mode enabled.
VIOLENCE IN SILENCE: Navigate the impound lot and reach the cemetary safe room without tripping any alarms.
WEATHERMAN: Survive the Hard Rain campaign.
WING AND A PRAYER: Defend yourself at the crashed airliner without taking damage.

20 Car Pile-up (20): As a Tank hit 20 Survivors with a car in the Crash Course campaign.
Crash-proof (15): Survive the Crash Course Campaign.
Jumpin' Jack Smash (25): Pounce a Survivor for 25 points of damage in the Crash Course campaign.
Quick Power (25): Restart the generator within 30 seconds of it shutting off in the Crash Course campaign.
Slippery Pull (25): Smoker pull a bile-covered Survivor until you hold him during Crash Course.
Smash Hit (25): Win a Versus campaign of Crash Course.
Tank Stumble (20): Stun a Tank with an explosion in the Crash Course campaign.
The Littlest Genocide (25): Kill 5,359 Infected in the Crash Course campaign.
Truck Stop (35): Your team wipes all Survivors after the escape vehicle has opened in Crash Course.
Wipefest (35): Your team incapacitates three Survivors within five seconds in the Crash Course campaign.